The present invention generally relates to a magnetic bubble memory device and in particular to a high-density magnetic bubble memory device using magnetic bubbles having a diameter not larger than 2 .mu.m.
FIG. 1 shows the configuration of a typical magnetic bubble memory device or chip. In FIG. 1, m denotes minor loops for storing information in the form of magnetic bubbles (hereinafter referred to simply as bubbles). RML denotes a read track for transferring information read out and WML denotes a write track for transferring information to be written. A detector D detects bubbles transferred for read-out and a generator G generates bubbles for write-in. A replicate gate R replicates the information from the minor loops m onto the read track RML. A swap gate S swaps the information on the write track WML with the information in the minor loops m. The minor loop m is usually composed of a propagation pattern of elements made of permalloy.
In conventional magnetic bubble memory devices, when high density is desired in such a manner that the period of elements in a propagation pattern composing minor loops is selected to be approximately 6 to 8 .mu.m so that the diameter of the available bubble (defined as a stripe width when a biasing field is zero) will be approximately 1.5 to 2 .mu.m, the bubble propagation characteristic becomes unstable. As a result, a biasing field margin large enough for practical use cannot be expected.
FIG. 2 shows a relationship between the bubble density .rho. of the minor loop and the biasing field margin .DELTA.H.sub.B for a permalloy device in which the bubble diameter is 1.8 .mu.m and the thickness of a magnetic layer defining the height of the bubble is 1.8 .mu.m. Variation of the biasing field margin when more and more bubbles are written into the minor loop is seen in FIG. 2. The bubble density of 100% refers to a state wherein bubbles are loaded in all addresses of the minor loop, i.e., a full load state.
It is apparent from FIG. 2 that the biasing field margin .DELTA.H.sub.B is sufficiently high when the bubble density .rho. is low but .DELTA.H.sub.B decreases as .rho. increases. The margin .DELTA.H.sub.B for the .rho. value of 100% is decreased to half as compared with the value in normal use and hence is insufficient for practical use.